Alcohols production by hydrogenation of carboxylic acids

ABSTRACT

Ethanol is produced from acetic acid or propanol is produced from propionic acid by contacting either acetic acid or propionic acid in the vapour phase with hydrogen at elevated temperature and a pressure in the range from 1 to 150 bar in the presence of a catalyst comprising as essential components (i) a noble metal of Group VIII of the Periodic Table of the Elements, and (ii) rhenium, optionally on a support, for example a high surface area graphitised carbon.

This application is a continuation of application Ser. No. 065,677,filed June 18, 1987, now abandoned, which is a continuation ofapplication Ser. No. 849,050, filed Apr. 7, 1986, now abandoned.

The present invention relates in general to the hydrogenation ofcarboxylic acids. In particular the present invention relates to aprocess for the hydrogenation of acetic and propionic acids in thepresence of a catalyst comprising a noble metal of Group VIII of thePeriodic Table of the Elements and rhenium to produce respectivelyethanol and propanol.

The hydrogenation of carboxylic acids to produce the correspondingalcohol using supported Group VIII noble metal catalysts is known from,for example, U.S. Pat. Nos. 4,524,225; 4,104,478; GB-A-No. 1534232;GB-A-No. 1551741 and EP-A-No. 147219. Of the aforesaid patents, allexcept GB-A-No. 1534232 relate to the hydrogenation of C₄ and highercarboxylic acids and, in common with GB-A-No. 1534232, to operation inthe liquid phase. Moreover, EP-A-No. 147219 represents an interveningpublication in the sense that it was published after the priority dateclaimed for the subject application on an application claiming anearlier priority date than the subject application.

GB-A-No. 1534232 relates to the production of alcohols by the catalytichydrogenation of carboxylic acids, including acetic acid and propionicacid, at elevated temperature and pressure in the presence of waterand/or solvents using as catalyst palladium/rhenium on a support, thepalladium to rhenium weight ratio of the catalyst being in the rangefrom 0.01 to 5:1. The process is operated at pressures in the range from50 to 1000 atmospheres. The only processes exemplified are thehydrogenation of C₄ and higher dibasic acids at very high pressures

We have found that operation of a Group VIII noble metal catalyst in theliquid phase suffers from the disadvantage that leaching of both rheniumand Group VIII noble metal from the catalyst can occur. Not onlyleaching of the catalytic metals but also undesirable leaching ofoxide-containing supports can occur. We have now suprisingly found thatoperation in the vapour phase provides high and comparatively long-livedcatalytic activity and selectivity at lower pressures than thosepreviously employed. Furthermore, operation in the vapour phasesubstantially overcomes the leaching problem associated with liquidphase operation.

Accordingly, the present invention provides a process for the productionof either ethanol from acetic acid or propanol from propionic acid whichprocess comprises contacting either acetic acid or propionic acid in thevapour phase with hydrogen at elevated temperature and a pressure in therange from 1 to 150 bar in the presence of a catalyst comprising asessential components (i) a noble metal of Group VIII of the PeriodicTable of the Elements, and (ii) rhenium.

In addition to the alcohol, the process of the invention generallyproduces the corresponding ester as a by-product, for example thehydrogenation of acetic acid generally also produces ethyl acetate andthe hydrogenation of propionic acid generally also produces propylpropionate. The proportion of the ester in the product may be increased,if desired, by for example operating at low conversions, for example atless than 50% conversion per pass, or by introducing an acidic functioninto the catalyst to promote `in situ` esterification. Alternatively,the proportion of alcohol may be increased, for example by co-feedingwater or by operating at very high conversions per pass.

Both acetic and propionic acids are commercially available in largetonnages and may be used in the process of the present invention intheir commercially available forms without further purification.Alternatively, they may be further purified if desired.

Hydrogen, too, is commercially available on a large scale and may beused with or without further purification.

The catalyst comprises a first component which is a noble metal of GroupVIII and a second component which is rhenium. For the avoidance ofdoubt, the noble metals of Group VIII are the metals osmium, palladium,platinum, rhodium, ruthenium and iridium. Of the aforesaid metals ofGroup VIII, palladium and ruthenium are preferred.

Preferably the catalyst further includes a support. Suitable supportsinclude high surface area graphitised carbons, graphites, silicas,aluminas and silica/aluminas, of which high surface area graphitisedcarbons and silicas are preferred. Preferred silica supports are thosehaving a high surface area, typically greater than 50 m² /g.

Particularly preferred supports are the high surface area graphitisedcarbons described in GB-A-No. 2136704. The carbon is preferably inparticulate form e.g. as pellets. The size of the carbon particles willdepend on the pressure drop acceptable in any given reactor (which givesa minimum pellet size) and reactant diffusion constrant within thepellet (which gives a maximum pellet size). The preferred minimum pelletsize is 0.5 mm and the preferred maximum is 10 mm, e.g. not more than 5mm.

The carbons are preferably porous carbons. With the preferred particlesizes the carbon will need to be porous to meet the preferred surfacearea charateristics.

Carbons may be characterised by their BET, basal plane, and edge surfaceareas. The BET surface area is the surface area determined by nitrogenadsorption using the method of Brunauer Emmett and Teller J. Am. Chem.Soc. 60,309 (1938). The basal plane surface area is the surface areadetermined from the heat of adsorption on the carbon of n-dotriacontanefrom n-heptane by the method described in Proc. Roy. Soc. A314 pages473-498, with particular reference to page 489. The edge surface area isthe surface area determined from the heat of adsorption on the carbon ofn-butanol from n-heptane as disclosed in the Proc. Roy. Soc. articlementioned above with particular reference to page 495.

The preferred carbons for use in the present invention have a BETsurface area of at least 100 m² /g, more preferably at least 200 m^(2/)g, most preferable at least 300 m² /g. The BET surface area ispreferably not greater than 1000 m² /g, more preferably not greater than750 m² /g.

The ratio of BET to basal plane surface area is preferably not greaterthan 4:1, more preferably not greater than 2.5:1. It is particularlypreferred to use carbons with ratios of BET to basal plane surface areaof not greater than 1.5:1.

It is preferred to use carbons with ratios of basal plane surface areato edge surface area of at least 10:1, preferably at least 100:1. It isnot believed that there is an upper limit on the ratio, although inpractice it will not usually exceed 200:1.

The preferred carbon support may be prepared by heat treating acarbon-containing starting material. The starting material may be anoleophillic graphite e.g. prepared as disclosed in GB No. 1,168,785 ormay be a carbon black.

However, oleophillic graphites contain carbon in the form of very fineparticles in flake form and are therefore not very suitable materialsfor use as catalyst supports. We prefer to avoid their use. Similarconsiderations apply to carbon blacks which also have a very fineparticle size.

The preferred materials are activated carbons derived from vegetablematerials e.g. coconut charcoal, or from peat or coal or fromcarbonizable polymers. The materials subjected to the heat treatmentpreferably have particle sizes not less than these indicated above asbeing preferred for the carbon support.

The preferred starting materials have the following characteristics: BETsurface area of at least 100, more preferably at least 500 m² /g.

The preferred heat treatment procedure for preparing carbon supportshaving the defined characteristics, comprise successively (1) heatingthe carbon in an inert atmosphere at a temperature of from 900° C. to3300° C., (2) oxidizing the carbon at a temperature between 300° C. and1200° C., (3) heating in an inert atmosphere at a temperature of between900° C. and 3000° C.

The oxidation step is preferably carried out at temperatures between300° and 600° C. when oxygen (e.g. as air) is used as the oxidisingagent.

The duration of the heating in inert gas is not critical. The timeneeded to heat the carbon to the required maximum temperature issufficient to produce the required changes in the carbon.

The oxidation step must clearly not be carried out under conditions suchthat the carbon combusts completely. It is preferably carried out usinga gaseous oxidizing agent fed at a controlled rate to avoid overoxidation. Examples of gaseous oxidising agents are steam, carbondioxide, and gases containing molecular oxygen e.g. air. The oxidationis preferably carried out to give a carbon weight loss of at least 10%wt based on weight of carbon subjected to the oxidation step, morepreferably at least 15% wt.

The weight loss is preferably not greater than 40% wt of the carbonsubjected to the oxidation step, more preferably not greater than 25% wtof the carbon.

The rate of supply of oxidizing agent is preferably such that thedesired weight loss takes place over at least 2 hours, more preferablyat least 4 hours.

Where an inert atmosphere is required it may be supplied by nitrogen oran inert gas.

Suitably the catalyst comprises from 0.1 to 10% by weight Group VIIInoble metal preferably from 0.5 to 5% by weight Group VIII noble metaland from 0.1 to 20% by weight rhenium, preferably from 1 to 10% byweight rhenium, the remainder of the catalyst comprising the support.

The catalyst may be further modified by the incorporation of a metal ormetals of Group IA, Group IIA or Group IVA, preferably by a metal ofGroup IA of the Periodic Table of the Elements. A suitable metal ispotassium. The amount of the modifying metal(s) may suitably be in therange from 0.1 to 20% by weight based on the total weight of thecatalyst. The addition of a modifying metal to the catalyst can have theadvantageous effect that carbon-carbon bond hydrogenolysis can besupressed to a greater or lesser extent during the hydrogenation,thereby improving the selectivity of the process to desired products.

The catalyst may be prepared by a variety of methods. One method ofpreparing the catalyst comprises impregnating the support with anaqueous solution of soluble compounds of rhenium and the Group VIIInoble metal which compounds are thermally decomposable/reducible to themetal and/or metal oxide.

Impregnation may be by way of co-impregnation or sequentialimpregnation, preferably by sequential impregnation. Sequentialimpregnation is preferably effected in the order Group VIII noble metalfollowed by rhenium.

A preferred method of producing a catalyst for use in the process of thepresent invention comprises the steps of:

(A) impregnating a support with a solution of a soluble Group VIII noblemetal compound thermally decomposable/reducible to Group VIII noblemetal and subsequently removing the solvent therefrom, and

(B) impregnating the Group VIII metal impregnated support with asolution in a solvent in which the Group VIII metal is substantiallyinsoluble of a soluble rhenium compound thermally decoaposable/reducibleto rhenium metal and/or an oxide and thereafter removing the solventtherefrom.

Water may suitably be employed as the solvent in step (A) and a loweralkanol, for example ethanol, may be used as the solvent in step (B).The production of a catalyst in the aforesaid manner can avoid thepalladium impregnated on the support in step (A) being leached to anyappreciable extent in step (B) of the process.

Another preferred method of producing a catalyst for use in the processof the present invention comprises the steps of:

(A') impregnating a support with a solution of a soluble Group VIIInoble metal compound thermally decomposable/reducible to the Group VIIInoble metal and subsequently removing the solvent therefrom,

(B') heating the Group VIII noble metal on the support, and

(C') impregnating the Group VIII noble metal impregnated support with asolution of a soluble rhenium compound thermally decomposable/reducibleto rhenium metal and/or oxide and thereafter removing the solventtherefrom.

The Group VIII noble metal on the support may suitably be heated in thepresence of either an inert gas, for example nitrogen, a reducing gas,for example hydrogen, or an oxygen-containing gas, for example air.Heating in the presence of an inert gas may suitably be accomplished atan elevated temperature in the range from 150° to 350° C. Heating in thepresence of a reducing gas may suitably be accomplished at an elevatedtemperature in the range from 100° to 350° C. Heating in the presence ofan oxygen-containing gas may suitably be accomplished at an elevatedtemperature in the range from 100° to 300° C., provided that when a highsurface area graphitised carbon is used as support the upper temperaturelimit is 200° C.

In this embodiment of the invention it is not necessary that a solventin which the Group VIII metal is substantially insoluble be used in step(C') of the process. Thus any suitable solvent may be used in steps (A')and (C') of the process. Suitable solvents include independently waterand alkanols.

An advantage of the heating step (step (B')) is that the noble metal ofGroup VIII is rendered less prone to leaching in step (C') of theprocess.

Preferably, a further step is interposed either between step (A) andstep (B) or between step (A') and step (B') wherein the Group VIII noblemetal impregnated support is dried, suitably by heating at a temperaturein the range from 50° to 150° C. It will be appreciated by those skilledin the art that this step may be incorporated into step (B'), ifdesired.

Suitable Group VIII noble metals which are decomposable/reducible to themetal include salts of the metals, for example carboxylates, nitratesand compounds in which the Group VIII noble metal is present in theanion moiety, for example ammonium tetrachloropalladate and ammoniumtetranitropalladate. Suitable rhenium compounds which aredecomposable/reducible to rhenium metal and/or oxide include dirheniumdecacarbonyl, ammonium perrhenate and rhenium heptoxide.

The metal of Group IA, Group IIA or Group IVA of the Periodic Table ofthe elements may be added to the catalyst composition at any pointduring its prepartion. Thus, the supported palladium/rhenium catalystmay be impregnated with a solution of a soluble compound of the metal.Alternatively, a soluble compound of the metal may be added to theco-impregnation solution or either of the sequential impregnationsolutions.

A preferred catalyst comprises palladium and rhenium supported on a highsurface area graphitised carbon of the type described in the aforesaidGB-A-No. 2136704. Contrary to the teaching of the aforesaid EP-A-No.0147219 (cf Comparison C) regarding unacceptable selectivity losses andundesirable productivity losses in the hydrogenation of maleic acid whenthe average palladium crystallite size is 100 Angstroms or less, we havefound that in the hydrogenation of acetic or propionic acids thecatalyst selectivity and productivity is substantially independent ofaverage palladium crystallite size in the range from 30 to 150Angstroms. We may therefore use catalysts in which the average palladiumcrystallite size is in the range from 30 to 99.9 Angstroms.

Before use in the process of the invention the catalyst is preferablyactivated by contact at elevated temperature with either hydrogen or ahydrogen/inert gas, for example nitrogen, mixture for a period of from 1to 20 hours. The elevated temperature may suitably be in the range from200° to 350° C. Alternatively, the catalyst may be activated by heatingto the reaction temperature in the presence of the reactants.

Whilst the precise nature of the catalyst on the support can not bedetermined with any degree of confidence, it is believed that the GroupVIII noble metal component is in the form of the elemental metal and therhenium component is in the form of the elemental metal and/or an oxidethereof.

The process of the invention may suitably be operated at an elevatedtemperature in the range from 100° to 350° C., preferably from 150° to300° C. The pressure may suitably be less than 50 bar.

The process may be operated batchwise or continuously, preferablycontinuously. The catalyst may be employed in the form of a fixed bed, amoving bed or a fluidised bed. The Gas Hourly Space Velocity forcontinuous operation may suitably be in the range from 50 to 50,000 h⁻¹,preferably from 2000 to 30,000 h⁻¹.

The process of the invention will now be further illustrated byreference to the following Examples.

CATALYST PREPARATION

Catalysts were prepared according to the procedures outlined below. Inthe procedures, HSAG carbon denotes high surface area graphitisedcarbon, prepared and characterised as follows:

The carbon used as support was prepared from a commercially availableactivated carbon sold by Degussa under the designation BK IV. Theactivated carbon was heat treated as follows. The carbon was heated fromroom temperature in a stream of argon to 1700° C. over a period of aboutone hour. When the temperature reached 1700° C. the carbon was allowedto cool in the stream of argon to 25° C. The carbon was then heated inair in a muffle furnace at approximately 520° C. for a time known fromexperience to give a weight loss of 20% wt. The carbon was then heatedin argon to between 1800° C. and 1850° C. in argon. The carbon wasallowed to cool to room temperature in an argon atmosphere. Theresulting graphite-containing carbon was then ground to 16-30 mesh BSS.

The resulting carbon had the following properties:

BET surface area: 710 m² /g

basal plane surface area: 389 m² /g

edge surface area: 2.3 m² /g

BET/basal surface area ratio: 1.83

basal plane/edge surface area ratio: 169

EXAMPLE 1

In the following procedures nominal loading is defined as weight ofmetal (not salt) added to the support expressed as a percentage of theweight of support.

A. An aqueous solution containing dissolved palladium nitrate andrhenium heptoxide (Re₂ O₇) was added to HSAG carbon. The water wasremoved on a rotary evaporator, and the resulting impregnated carbon wasthen dried at 100° C. in a vacuum oven overnight. The amounts of thevarious components were chosen to give four catalysts with nominalloadings as follows: A1-2.5% Pd, 5% Re; A2-2.5% Pd, 2% Re; A3-2.5% Pd,10% Re; A4-5% Pd, Re excluded from the preparation.

B. The procedure used in the preparation of catalyst A was followed,except that an appropriate amount of ammonium perrhenate was usedinstead of Re₂ O₇, and the amounts of components were chosen to givefour catalysts with nominal loadings as follows:

B1-5% Re, 2.5% Pd; B2-5% Re, 10% Pd; B3-5% Re, 0.5% Pd; B4-5% Re, Pdexcluded.

C. An aqueous solution of palladium nitrate was added to HSAG carbon,the solvent was removed on a rotary evaporator, and the resultingimpregnated carbon catalyst dried overnight at 100° C. in a vacuum oven.The catalyst was then cooled and transferred to a glass tube, and wasthen heated in a stream of hydrogen from ca 30° to 280° C. over a periodof six hours. After ten hours at 280° C., the catalyst was cooled underhydrogen, and then purged for several hours with nitrogen.

The palladium on carbon was then mixed with an aqueous solution of Re₂O₇, the solvent again removed on a rotary evaporator, and the catalystdried overnight at 100° C. in a vacuum oven. The amounts of palladiumnitrate and rhenium heptoxide were chosen to give nominal loadings of2.5% Pd and 5% Re in the final catalyst.

D. The procedure used in the preparation of catalyst C was repeated,except that prior to impregnation of rhenium, the palladium impregnatedcarbon catalyst was treated in nitrogen at 300° C. instead of hydrogenat 280° C.

E. The procedure used in the preparation of catalyst C was repeated,except that the hydrogen treatment step prior to impregnation of rheniumwas replaced by an air treatment step as follows. The palladiumimpregnated carbon was heated from 20° to 180° C. in flowing air oversix hours, and held at 180° C. for four hours, before cooling in air to30° C.

F. This catalyst was prepared according to procedure C except that afterdrying, the palladium on carbon catalyst was not heated in hydrogen, andthe solvent used for the impregnation of rhenium was ethanol instead ofwater.

G. Procedure C was used, except that immediately before the rheniumimpregnation stage, the reduced palladium on carbon catalyst was treatedin flowing nitrogen by heating from 30° C. to ca 650°-700° C. over threehours, holding at 650°-700° C. for a further sixteen hours, and thencooling to 30° C. The effect of this additional step was to increase thepalladium crystallite size (as measured by XRD) from 30 Angstrom(catalyst from procedure C) to 150 Angstrom (catalyst from thisprocedure).

H. Catalysts containing ruthenium, rhenium and potassium were preparedas follows. HSAG carbon was mixed with a solution containing rutheniumtrichloride and ammonium perrhenate, the solvent was removed on a rotaryevaporator, and the resulting catalysts dried ca 100° C. overnight in avacuum oven. The catalyst was then heated in flowing hydrogen from ca30° to 300° C. over two hours, held at 300° C. for one hour, then cooledunder hydrogen and purged with nitrogen. The reduced catalysts were theniapregnated with potassium from an aqueous solution of potassiumacetate. The amounts of the various ingredients were adjusted to givefour catalysts with nominal loadings as follows: H1-5% Re, 5% Ru, (Kexcluded); H2-5% Re, 5% Ru, 10% K; H3-5% Ru, 5% K (Re excluded); H4-5%Ru (Re and K excluded).

I. A catalyst containing ruthenium and rhenium was prepared according toprocedure C., except that ruthenium nitrosyl nitrate replaced palladiumnitrate, the ruthenium on carbon catalyst was dried at 120° C. not 100°C., and was then heated in hydrogen to 300° C. at 4° C./minute, and heldat 300° C. for one hour. The amounts of the ingredients were chosen togive nominal loadings of 1% ruthenium and 10% rhenium.

J. A ruthenium/rhenium catalyst was prepared as in procedure I, exceptthat rhenium was impregnated first.

K. Procedure A was used except that HSAG carbon was replaced by Davison57 silica, aamonium tetrachloropalladate was used instead of palladiumnitrate, and only one catalyst containing nominally 2.5% Pd and 5% Rewas prepared.

L. Procedure C was used for the preparation of a catalyst containingplatinum and rhenium. Tetrammine platinous hydroxide replaced palladiumnitrate, and the nominal loadings were 1% Pt and 5% Re.

CATALYST TESTING

For experiments at pressures in the range 1-11 barg, 2.5 mls of catalystwas loaded into a corrosion resistant stainless steel tube of internaldiameter 6-7 mm, and the reactor tube assembly placed in a tubularfurnace. The catalyst was then activated by heating at atmosphericpressure in a stream of hydrogen to either 280° or 300° C. over a twohour period, and then holding at the final temperature for one hour.After activation, the catalyst was cooled in hydrogen to the desiredreaction teaperature. A mixture of carboxylic acid vapour and hydrogenwas then passed over the catalyst, and pressure was adjusted to therequired value by means of a back-pressure regulator. Thevapour/hydrogen mixture was formed in a vapourising zone, to whichacetic acid liquid and hydrogen gas were separately metered. The productvapours and gases leaving the reactor were sampled on-line and analysedby gas-liquid chromatography (glc).

For experiments conducted at 11-50 barg, a similar procedure andapparatus was used, except that the tube had internal diameter 10 mm, upto 10 mls of catalyst was employed, and products were passed to acondenser, and gas and liquid products were analysed separately, againby glc.

In both procedures, temperature was measured by means of a thermocoupleinserted into the catalyst bed.

The product mixtures typically contained the appropriate alcohol andester (the latter formed by esterification of alcohol with unreactedacid), together with traces of the appropriate dialkyl ether, andaldehyde, and by-product methane, ethane and (with propionic acid only)propane. In general, with carbon and silica supported catalysts, themain product is alcohol, especially at high conversions.

For the purposes of the Examples, conversions and selectivities havebeen calculated as respectively, the proportion of carboxylic acidhydrogenated, and the proportion of the hydrogenated carboxylic acidwhich is not converted into alkane by-product. Thus, selectivity denotesthe ability of the catalyst to carry out hydrogenation withoutalkanation. In all examples (unless stated otherwise) only trace amounts(≦2%) of dialkyl ether and aldehyde are formed.

DEFINITIONS

WHSV=Weight Hourly Space Velocity=kg liquid feed per kg catalyst perhour.

LHSV=Liquid Hourly Space Velocity=liters liquid feed per liter ofcatalyst per hour.

Productivity=kg acid converted per kg catalyst per hour.

EXAMPLES 2-7

Acetic acid was hydrogenated over the catalysts prepared in procedure AExample 1, and procedure C Example 1. The WHSV was ca 1.1 (LHSV=0.35),the ratio hydrogen to acetic acid was ca 11:1 molar, and the pressurewas 10.3 barg. In each case the catalyst was activated at 300° C.,except for the catalyst of Example 7 (C), which was activated at 280° C.The results are collected in Table 1. Steady catalyst activity wasobserved in all cases. No deactivation was observed over run periods ofup to 24 hours.

                  TABLE 1                                                         ______________________________________                                                                   Conversion                                                                             Selectivity                               Example Catalyst  T/°C.                                                                           (%)      (%)                                       ______________________________________                                        2       A1        222      27.2     91                                        3       A1        202      15.0     90                                        4       A2        202      6.3      93.6                                      5       A3        201      38.2     95.9                                      6       A4        200      0.6      30.4                                      7       C         217      52.1     93                                        ______________________________________                                    

The results show the benefit of sequential impregnation of Pd and Re(Example 7), and the poor performance of catalyst A4 (Example 6), whichcontains only palladium, and is not a catalyst according to theinvention.

EXAMPLES 8-13

The same procedure as in Examples 2-7 was followed, but using thecatalysts prepared according to procedure B Example 1. All catalystswere activated at 300° C. Results are presented in Table 2.

                  TABLE 2                                                         ______________________________________                                                                   Conversion                                                                             Selectivity                               Example Catalyst  T/°C.                                                                           (%)      (%)                                       ______________________________________                                         8      B1        180      15.4     97.0                                       9      B1        210      37.5     95.1                                      10      B1        239      69.0     89.0                                      11      B2        210      45.0     95.0                                      12      B3        210      18.5     96.9                                      13      B4        210      13.7     97.6                                      ______________________________________                                    

The Catalyst of Example 13 is not according to the present invention,and is included for the purposes of comparison.

EXAMPLES 14-17

The catalyst prepared by procedures C, D, E and F of Example 1 werecompared in the hydrogenation of acetic acid. The procedure of Examples2-7 was followed, except that the WHSV was ca 4 (LHSV=1.34), and theratio hydrogen to acetic acid was 9:1 molar. The catalysts wereactivated at 280° C. before use, and the reaction temperature was228°-230° C. Results are collected in Table 3.

                  TABLE 3                                                         ______________________________________                                                             Productivity                                                                             Selectivity                                   Example  Catalyst    (kg/kg cat/h)                                                                            (°C.)                                  ______________________________________                                        14       C           1.2        92.2                                          15       D           1.3        92.1                                          16       E           1.1        87.0                                          17       F           0.95       94.5                                          ______________________________________                                    

The results show that within experimental error, catalysts of similarhigh activity may be generated using a range of sequential impregnationtechniques.

EXAMPLE 18

The procedure of Examples 14-17 was repeated using the catalyst preparedaccording to procedure G Example 1. The productivity was found to be 1.0kg/kg cat/h, with 92.7% selectivity. Within experimental error, theseresults are similar to those obtained in Example 14, even though thecatalyst of this Example has Pd crystallites (as determined by XRD) ofaverage size 150 Angstrom, whereas that of Example 14 has an average Pdcrystallite size of only 30 Angstrom. The results show that nosignificant losses of activity and selectivity result when catalystscontaining small Pd crystallites of <100 Angstrom are employed incontrast to the teaching of EP-A-No. 147219 (Comparison C).

EXAMPLE 19

The catalyst prepared by procedure C was tested in acetic acidhydrogenation at 50 barg and 227° C. The WHSV was 15, and the ratiohydrogen:acetic acid was 9:1 molar. The catalyst was activated at 280°C.

The acetic acid conversion was 40%, with 96% selectivity. Thiscorresponds to a productivity of 6 kg/kgcat/h acetic acid converted.Under similar conditions but with WHSV=3.6, conversion was 74% with 96%selectivity.

EXAMPLES 20-24

The catalysts prepared by procedure H were tested in the hydrogenationof acetic acid. The catalysts were activated at 300° C. The WHSV was ca1.1 (LHSV=0.35), and the ratio hydrogen to acetic acid was 11:1 molar.Results are collected in Table 4.

                  TABLE 4                                                         ______________________________________                                                                      Conversion                                                                            Selectivity                             Example                                                                              Catalyst P/barg  T/°C.                                                                        (%)     (%)                                     ______________________________________                                        20     H1       5       200   46      38                                      21     H2       5       202   43      53                                      22     H2       10      194   54      58.5                                    23     H3       10      203   35.2    8.7                                     24     H4       5       201   22.3    5.9                                     ______________________________________                                    

The results show the beneficial effect of potassium in improvingselectivity, and that catalysts H3 and H4 which are not according to thepresent invention, show very poor performance.

EXAMPLES 25-28

Catalysts prepared by procedures I and J of Example 1 were examined inthe hydrogenation of propionic acid. The procedure of Examples 2-7 wasrepeated, except that only 2 mls of catalyst was employed, LHSV=1, theratio of propionic acid to hydrogen was 1:10 molar, the pressure was 9barg, and the catalyst were activated at 280° C. Results are collectedin Table 5. In each case, the concentration of aldehyde in the productwas greater than the trace amounts encountered in other Examples.Independent selectivities to aldehyde are therefore reported.

                  TABLE 5                                                         ______________________________________                                        Exam- Cata-           Conversion                                                                            Selectivity                                                                           Selectivity                             ple   lyst    T/°C.                                                                          (%)     (%)     (% aldehyde)                            ______________________________________                                        25    I       202     22.5    97      4                                       26    I       223     32.0    94      3                                       27    J       201     12.5    97      5                                       28    J       222     23.0    96      5                                       ______________________________________                                    

The results show that sequential impregnation of Ru then Re yieldsbetter catalysts than sequential impregnation of Re then Ru.

EXAMPLES 29 and 30

The catalysts prepared by procedure K Example 1 were tested in thehydrogenation of acetic acid. The procedure of Examples 2-7 was adopted,except that the catalyst of Example 30 was activated at 450° C., andthat of 29 at 300° C. Results are collected in Table 6.

                  TABLE 6                                                         ______________________________________                                                                   Conversion                                                                             Selectivity                               Example Catalyst  T/°C.                                                                           (%)      (%)                                       ______________________________________                                        29      K         209      12.2     91.7                                      30      K         210      10.5     95.3                                      ______________________________________                                    

EXAMPLE 31

The catalyst prepared by procedure L was eaployed for the hydrogenationof acetic acid, according to the procedure of Examples 14-17. Theconversion was 11.0% (productivity 0.5 kg/kg cat/h converted) with 93.8%selectivity.

EXAMPLE 32

The catalyst prepared according to procedure B1 was used for the liquidphase hydrogenation of acetic acid. 1.01 g of the powdered catalyst wascharged to a 100 ml stainless steel autoclave, along with 50.2 g ofacetic acid. The autoclave was flushed and then pressurised withhydrogen to 100 barg, and heated with stirring to 200° C., at whichtemperature it was held for 6.0 hours. After cooling, the liquid phaseproduct was removed and filtered, and analysed both for organic productsand rhenium and palladium metals. The final pressure after cooling was50 barg.

The product was found to contain 27.9% wt ethyl acetate and 2% wtethanol (corresponding to a productivity of 1.5 kg/kg cat/h converted byhydrogenation). In addition, 16% of the rhenium and 0.06% of thepalladium originally on the catalyst was found to have leached intosolution.

This example demonstrates that considerable leaching of rhenium canoccur in the liquid phase hydrogenation of acetic acid. This is incontrast to reactions carried out in the gas phase, where no detectableloss of rhenium occurs.

This is not an example according to the present invention because it wascarried out in the liquid phase. It is included only for the purpose ofcomparison.

We claim:
 1. A process for the production of either ethanol from aceticacid or propanol from propionic acid which process comprises contactingeither acetic acid or propionic acid in the vapour phase with hydrogenat elevated temperature and a pressure in the range from 1 to 150 bar inthe presence of a catalyst comprising as essential components (i) anoble metal of Group VIII of the Periodic Table of the Elements, and(ii) rhenium.
 2. A process according to claim 1 wherein the noble metalof Group VIII is palladium.
 3. A process according to claim 1 whereinthe noble metal of Group VIII is ruthenium.
 4. A process according toclaim 1 wherein the catalyst is supported.
 5. A process according toclaim 4 wherein the support is a high surface area graphitised carbon.6. A process according to claim 4 wherein the support is a silica.
 7. Aprocess according to claim 1 wherein the catalyst is modified byincorporation of a metal of Group IA of the Periodic Table of theElements.
 8. A process according to claim 7 wherein the modifying metalis potassium.
 9. A process according to claim 1 wherein the catalyst isactivated before use by contact at elevated temperatsure with eitherhydrogen or a hydrogen/inert gas mixture at a temperature in the rangefrom 200 ° to 350° C. for a period of from 1 to 20 hours.
 10. A processaccording to claim 1 wherein the catalyst is activated by heating to thereaction temperature in the presence of reactants.
 11. A processaccording to claim 1, wherein the catalyst comprises palladium andrhenium supported on a high surface area graphitised carbon wherein theaverage palladium crystalline size is in the range from 30 to 99.9Angstroms.
 12. A process for the production of either ethanol fromacetic acid or propanol from propionic acid which process essentiallyconsists of contacting either acetic acid or propionic acid in thevapour phase with hydrogen at elevated temperature and a pressure in therange from 1 to 150 bar in the presence of a catalyst which essentiallyconsists of as essential components (i) palladium or ruthenium, (ii)rhenium, and (iii) potassium,wherein the catalyst is supported on a highsurface area graphitised carbon, wherein there is from 0.1 to 10% byweight of component (i), wherein there is from 0.1 to 20% by weightcomponent (ii), and wherein there is from 0.1 to 20% by weight component(iii), with these component weights being based upon the total weight ofcatalyst.